One of the concerns about using six sigma is the volume of production. There are two parts to this concern. The immediate reaction is that the 3.4 PPM defect rate associated with six sigma might imply that the volume of production has to be very large in order to properly assess this high level of quality. The other concern is that the tools of six sigma used for quality control and defect rate prediction might not apply because of the difficulty of properly obtaining statistical information such as the standard deviation of the manufacturing variability of the production process. Low-volume industries including defense, aerospace, and medical, as well as their suppliers, share these concerns.
Several statistical tools will be discussed in this chapter in order to allow for the use of six sigma in low-production environments, with minimum uncertainties. They are based on sampling theory and distribution, and the relationships between samples and populations. These tools are:
1. Process average and standard deviation calculations for samples and populations. Section 5.1 will discuss the sample probability distribution and its relationship to the parent population distribution. It gives examples of determining population standard deviation and error based on sample sizes,
2. Determining process capability. Section 5.2 will discuss the amount of data required to properly determine process capability. The data volume is important in increasing the accuracy and the amount of effort necessary to correct the design or the manufacturing process to meet the process capability goals. This section will also examine moving range control charts as a means of controlling quality in low-volume production.
3. Determining gauge capability. The use of gauge repeatability and reproducibility (GR&R) to quantify measurement variability will be presented in Section 5.3. In addition, The relationship of GR&R
4. to six sigma concepts and calculations will be examined.
5. Determining short- and long-term process capability. Section 5.4 will discuss the issues of determining process capability during the different stages of the product lifecycle, beginning with multiple specifications of the product and prototype quantities manufactured, and continuing with production volume. The strategies of setting different quality expectations during prototype versus volume production will also be examined.